Bottom-up and/or top-down growth methods that allow easy manipulation of chemical composition and morphology of heterostructured 1-D nanostructures, such as core/shell nanowires, are a fabrication challenge (Mann, S. Nat. Mater. 2009, 8, 781-792; Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834-2860; Fu, J. X.; He, Y. P.; Zhao, Y. P. IEEE Sens. J. 2008, 8, 989-997; Bierman, M. J.; Jin, S. Energy Environ. Sci. 2009, 2, 1050-1059; Eymery, G.; Biasiol, J.; Kapon, E.; Ogino, T. C. R. Physique, 2005, 6, 105-116). The interfaces in such heterostructured and semiconducting nanowires (e.g., oxides, nitrides, and phosphides) facilitate rapid charge transport and exhibit unique electronic and photonic properties (Grimes, C. A.; Varghese, O. K.; Ranjan, S. K. Light, water, and hydrogen: The solar generation of hydrogen by water photoelectrolysis; Springer: New York, 2008; Baxter, J. B.; Aydil, E. S. Appl. Phys. Lett. 2005, 86, 053114; Zhou, W.; Cheng, C.; Liu, J.; Tay, Y. Y.; Jiang, J.; Jia, X.; Zhang, J.; Gong, H.; Hng, H. H.; Fan H. J. Adv. Funct. Mater. 2011, 21, 2439-2445; Hill, J. J.; Banks, N.; Haller, K.; Orazem, M. E.; Ziegler, K. J. Nano Lett. 2011, 133, 18663-18672; Musin, R. N.; Wang, X. Q. Phys. Rev. B 2005, 71, 155318). For example, core/shell nanowire heterostructures can allow for multi-level light-matter interaction with charge transport directed across the thin shell (radially) and rapid charge conduction through the core (longitudinally) (Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834-2860; Bierman, M. J.; Jin, S. Energy Environ. Sci. 2009, 2, 1050-1059; Yang, D.; Liu, H.; Zheng, Z.; Yuan, Y.; Zhao, J. C.; Waclawik, E. R.; Ke, X.; Zhu, H. J. Amer. Chem. Soc. 2009, 131, 17885-17893). Such interaction is not possible with single-component nanowires and makes nanowire heterostructures extremely attractive for water splitting, CO2 photocatalytic reduction (Pan, P, W.; Chen, Y. W. Catal. Comm. 2007, 8, 1546-1549), and solar energy harvesting (Bierman, M. J.; Jin, S. Energy Environ. Sci. 2009, 2, 1050-1059; Hill, J. J.; Banks, N.; Haller, K.; Orazem, M. E.; Ziegler, K. J. Nano Lett. 2011, 133, 18663-18672; Barreca, D.; Fornasiero, P.; Gasparotto, A.; Gombac, V.; Maccato, C.; Montini, T.; Tondello, E. Chem. Sus. Chem. 2009, 2, 230-233).
Nanowire heterostructures composed of oxides are of interest for a wide array of applications and can be synthesized using various methods including solution synthesis, gas phase growth (physical vapor deposition (PVD) and chemical vapor deposition (CVD)), air oxidation, and flame synthesis (Kemell, M.; Harkonen, E.; Pore, V.; Ritala, M.; Leskela, M. Nanotechnology 2010, 21, 035301; Zhang, H.; Luo, X.; Xu, J.; Xiang, B.; Yu, D. J. Phys. Chem. B 2004, 108, 14866-14869; Chopra, N. Mater. Technol. 2010, 25, 212-230; Shahida, M.; Shakira, I.; Yang, S. J.; Kang, D. J. Mater. Chem. Phys. 2010, 124, 619-622; Chueh, Y. L.; Hsieh, C. H.; Chang, M. T.; Chou, L. J.; Lao, C. S.; Song, J. H.; Gan, J. Y.; Wang, Z. L. Adv. Mater. 2007, 19, 143-149; Feng, Y.; Cho, I. S.; Rao, P. M.; Cai, L.; Zheng, X. Nano Lett. 2012, Article ASAP. DOI: 10.1021/n1300060b; Chun, J.; Lee, J. Eur. J. Inorg. Chem. 2010, 27, 4251-4263). Typically, a core nanowire is coated with a layer of second component with specific thickness and composition (Kemell, M.; Harkonen, E.; Pore, V.; Ritala, M.; Leskela, M. Nanotechnology 2010, 21, 035301; Zhang, H.; Luo, X.; Xu, J.; Xiang, B.; Yu, D. J. Phys. Chem. B 2004, 108, 14866-14869; Shahida, M.; Shakira, I.; Yang, S. J.; Kang, D. J. Mater. Chem. Phys. 2010, 124, 619-622; Tak, Y.; Hong, S. J.; Lee, J. S.; Yong, K. J. J Mater. Chem. 2009, 19, 5945-5951). The solution routes have their advantages; they are simple, cheap, and scalable (Tak, Y.; Hong, S. J.; Lee, J. S.; Yong, K. J. J Mater. Chem. 2009, 19, 5945-5951; Shi, W.; Chopra, N. J. Nanopart Res. 2011, 13, 851-868). Dispersion of metal salts onto nanowire and subsequent thermal decomposition has shown potential to result in nanowire heterostructures organized in vertical arrays or horizontally-suspended architectures (Shi, W.; Chopra, N. J. Nanopart Res. 2011, 13, 851-868; Zhao, X.; Wang, P.; Li, B. Chem. Commun. 2010, 46, 6768-6770). Flame synthesis is a scalable approach for fabricating oxide nanowire heterostructures but the challenge is to control the growth of uniform and thin oxide shells around core nanowires (Feng, Y.; Cho, I. S.; Rao, P. M.; Cai, L.; Zheng, X. Nano Lett. 2012, Article ASAP. DOI: 10.1021/n1300060b). Gas phase techniques are well suited but necessitate understanding materials-specific thermodynamics and processes to avoid formation of continuous films on the surfaces (Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era. Lattice Press: Sunset Beach, 1999). Although atomic level control of stoichiometric ratio and perfect site-selective deposition can be obtained for nanowire heterostructures using CVD and pulsed laser deposition (PLD) (Li, L.; Koshizaki, N. J. Mater. Chem. 2010, 20, 2972-2978; Han, S.; Zhang, D.; Zhou, C. Appl. Phys. Lett. 2006, 88, 133109), high cost and low through-put remains a problem. As compared to the abovementioned methods, sputter deposition is a conventional, scalable, and cost-effective technique for assembling oxide nanostructures (e.g., nanowires) on the substrate, without patterning or using templates (LaForge, J. M.; Taschuk, M. T.; Brett, M. J. Thin Solid Films 2011, 519, 3530-3537; Huang, J. H.; Chen, C. Y.; Lai, Y. F.; Shih, Y. I.; Lin, Y. C.; He, J. H.; Liu, C. P. Cryst. Growth Des. 2010, 10, 3297-3301). This approach also holds significant promise for developing complex nanowire heterostructures (Noh, J. S.; Lee, M. K.; Ham, J.; Lee, W. Nanoscale Res. Lett. 2011, 6, 598).
Among various oxides, CuO and Co3O4 photocatalysts are of particular interest (Barreca, D.; Fornasiero, P.; Gasparotto, A.; Gombac, V.; Maccato, C.; Montini, T.; Tondello, E. Chem. Sus. Chem. 2009, 2, 230-233; Shi, W.; Chopra, N. J. Nanopart Res. 2011, 13, 851-868; Zhao, X.; Wang, P.; Li, B. Chem. Commun. 2010, 46, 6768-6770; Shi, W.; Chopra, N. Mater. Res. Soc. Res. Proc. 2010, 1256, 1256-N10-03; Gasparotto, A.; Barreca, D.; Bekermann, D.; Devi, A.; Fischer, R. A.; Fornasiero, P.; Gombac, V.; Lebedev, O. L.; Maccato, C.; Montini, T.; Tendeloo, G. V.; Tondello, E. J. Amer. Chem. Soc. 2011, 133, 19362-19365). These oxides are stable and can result in narrow to wide band gap energies depending on their dimensions/morphologies (Grimes, C. A.; Varghese, O. K.; Ranjan, S. K. Light, water, and hydrogen: The solar generation of hydrogen by water photoelectrolysis.; Springer: New York, 2008; Barreca, D.; Fornasiero, P.; Gasparotto, A.; Gombac, V.; Maccato, C.; Montini, T.; Tondello, E. Chem. Sus. Chem. 2009, 2, 230-233). Such oxide-based photocatalysts are also considered as potential replacements for precious metals (Maeda, K.; Ohno, T.; Domen, K. Chem. Sci. 2011, 2, 1362-1368). In addition, semiconducting CuO nanowires can be grown in a simple and environment-friendly method (Jiang, X.; Herricks, T.; Xia, Y. Nano Lett. 2002, 2, 1333-1338). They can survive multiple processing steps and have the ability to combine with other material systems. All these characteristics make them interesting base materials for nanowire heterostructures (Feng, Y.; Cho, I. S.; Rao, P. M.; Cai, L.; Zheng, X. Nano Lett. 2012, Article ASAP. DOI: 10.1021/n1300060b; Jiang, X.; Herricks, T.; Xia, Y. Nano Lett. 2002, 2, 1333-1338; Kaito, C.; Nakata, Y.; Saito, Y.; Naiki, T.; Fujita, K. J. Cryst. Growth 1986, 74, 469-479).
It was reported that CuO nanowires-Co3O4 nanoparticles heterostructures have unique photoactivity under low power (8 W) illumination lamp with organic dye degradation efficiencies as high as 17% compared to pristine CuO nanowires (Shi, W.; Chopra, N. J. Nanopart Res. 2011, 13, 851-868). The synthesis approach for these heterostructures involved wet-chemical coating of CuO nanowires with cobalt salt and thermal decomposition of the latter to obtain Co3O4 nanoparticles (Shi, W.; Chopra, N. J. Nanopart Res. 2011, 13, 851-868). However, this approach limited the growth of Co3O4 in the form of well-dispersed nanoparticles or islands on the CuO nanowire surface. Furthermore, such a processing method does not necessarily allow for manipulation of Cu and Co content in the heterostructures. What are needed are different processing methods that provide tenability of heterostructure morphology and composition. The methods and compositions disclosed herein address these and other needs.